Up-to-date fingerprint imaging systems using fingerprint image transfer into electronic data usually apply the known contact method to create a fingerprint pattern. A surface topography of a finger is approximated by a series of ridges with intermediate valleys. When a finger is applied to a surface of a transparent optical plate or prism, the ridges contact the optical plate while the valleys do not and instead serve to form the boundaries of regions of air and/or moisture.
The finger to be imaged is illuminated by a light source located below or near the optical plate. Imaging light from the light source is incident on the surface of the optical plate at an angle of incidence measured with respect to a normal to that surface. Imaging light reflected from the surface is detected by an imaging system that includes some form of a detector.
Components of a typical fingerprint imaging system are oriented so that an angle of observation (defined to be an angle between an optical axis of the imaging system and the normal to the optical plate surface) is greater than a critical angle for the interface between the surface and air at the surface. The critical angle at the surface/air interface is defined as the smallest angle of incidence for which imaging light striking the surface/air interface is totally internally reflected (TIR) within the optical plate. Therefore, the critical angle at the surface/air interface depends on the index of refraction of the air and the optical plate. Another constraint for the angle of observation arises because there is incentive to observe the image at the smallest practical angle of observation, as this reduces distortion due to object tilting. Therefore, the angle of observation is typically chosen to be close to, but greater than the critical angle at the surface/air interface.
A livescan imaging system may be configured to capture four-finger slap, single-finger slap, and rolled fingerprint images. Conventional fingerprint imaging systems of this type may provide 500 pixels per inch (ppi) images. However, it is also desirable to provide more detailed images such as 1,000 ppi images.
Conventional fingerprint imaging platforms use monochrome charged coupled device (CCD) imagers, monochromatic light sources, and anamorphic correcting optics to map an object plane to an image plane. CCD images and electronics, however, are expensive. Optics and opto-mechanics are also expensive, and a monochromatic light source produces light of only one color.
Another platform uses dual, small-format, low frame rate (about 4–5 frames per second (fps)), 1.3 megapixel complementary metal-oxide semiconductor (CMOS) color imagers, a monochromatic light source and monochromatic optics. The object plane is split with each half mapped to one of the pair of imagers. However, performance is not improved relative to other, conventional designs. Low frame rates also produce artifacts and the use of a monochromatic light source limits signal-to-noise in blue and red pixels. Additionally, relatively low imager pixel count limits the contrast transfer function (CTF).
Other livescan systems use two separate imaging chains, one for rolled images and one for four-finger slaps. CCD imagers are employed. Illumination is monochromatic. The four-finger slap imager employs a CMYG (cyan-magenta-yellow-green) color matrix CCD. The four-finger slap imager uses a red monochromatic light source which strongly stimulates the magenta and yellow pixels but weakly (if at all) the green and cyan pixels. Such weak pixel performance must be compensated for by strong equalization producing a pixel dependent noise pattern, or by interpolating strong pixel values to create or enhance weak pixel values. This technique results in inferior performance. Also, the four-finger slap imager provides a low frame rate (about 4 fps), yielding motion artifacts when the fingers move while forming the image.
Still other systems use a light pipe illumination scheme. However, the surface to be illuminated is small (1.6×1.5 inches) and the system is monochrome using light having a wavelength of about 650 nanometers (nm). One such system employs object plane telecentric optics. Another uses a pair of cylinder lenses to provide anamorphic distortion to map the object plane format to the image plane format using the maximum number of pixels (non-square pixels). Another system makes use of a prism pair to anamorphically distort the image in the vertical domain to map the object plane exactly to the image plane to accomplish exactly 500 ppi, square pixels.